1) Field of the Invention
The present invention relates to a cleaning device that scraps off the toner on the surface of the image carrier of an image forming apparatus.
2) Description of the Related Art
Image forming apparatuses such as printers, facsimiles, and copiers those use electrophotography to form images are widely used. Such an image forming apparatus has a cleaning device that scraps off toner remaining on a surface of an image carrier after the toner is transferred. Sometimes the toner is collected and recycled.
The cleaning device includes a cleaning blade. This cleaning blade is generally made of an elastic material such as rubber. This is because, the cleaning blade made of rubber has a simple structure and it can scrap off the toner very effectively.
It is known, that better image quality is achieved if the particles of the toner are spherical (hereafter, “spherical toner”). Therefore, the spherical toner is becoming popular. The spherical toner is produced by polymerization. To improve the image quality, the approaches are to reduce the particle size of the toner (hereafter, “toner size”) or to use more spherical particles.
However, if the toner size is too small or if the toner is perfectly spherical, the cleaning device can not completely scrap off the toner. If the toner remains on the image carrier, the image quality degrades, and the toner flies here and there inside the device. Particularly, the toner remaining on the image carrier gets stick to a charger that electrically charges the image carrier.
The spherical toner with particles having sphericity of 1 or close to 1 is particularly difficult to scrap off. Even if the average sphericity of the particles is 0.95 or less, the toner includes particles of sphericity of 1 or close to 1 and makes the toner difficult to scrap off.
Environmental temperature also affects the cleaning capability. The image forming apparatuses are installed in places where the temperature is between 10° C. and 30° C. However, the cleaning capability worsens as the temperature drops.
JP-A No. 2001-188452 discloses in detail why the fine toner or spherical toner is difficult to scrap off. In general the reasons are as follows.
When the blade made of rubber is used, the edge of the rubber blade bends as the blade slides on the image carrier. Consequently, a wedge-shaped space is formed between the blade and the image carrier. Toner enters into this space. More toner enters into this space if the toner is fine. Moreover, more toner enters into this space if the particles are spherical.
Once the toner enters into this space, it remains there almost permanently. In other words, a “non-flow region” is formed in this space. When the frictional resistance between the toner in the non-flow region and the image carrier is small and the toner smoothly slide over the image carrier, faulty cleaning does not occur. However, if the friction increases, due to say separation of an external additive from the toner because when the toner slides against the image carrier, the spherical particles start rolling because the frictional force of spherical particles is smaller than irregular particles. As a result, some of the particles come out of the non-flow region and disadvantageously remain at the surface of the image carrier.
Japanese Patent Application Laid Open (JP-A) No. 2001-188452 discloses a technology to solve this problem. The cleaning device disclosed in this publication includes a blade and a brush. The blade scrapes off the toner on the photoreceptor. The brush is disposed on the upstream side, with respect to the direction in which the photoreceptor moves, of the blade. This brush pulverizes the remaining toner and creates fine-grained toner particles.
JP-A No. 2000-267536 also discloses a technology to solve the problem described above. The image forming apparatus disclosed in this publication includes a toner image carrier and a transfer device. The toner image carrier rotates so that the surface, which carries the toner image formed with the spherical toner, of the toner image carrier passes through a region (hereafter, “transfer region”) in which the toner image is transferred and a region (hereafter, “cleaning region”) in which the toner is scraped off. In the transfer region, the transfer device transfers the toner image onto a transfer material. In the cleaning region, a blade having an edge, which makes a contact with the surface of the image carrier, scraps the toner. A cleaner is applied to the edge of the blade. This cleaner is a mixture of powdered lubricant and toner having irregular particles and an average particle size smaller than that of the spherical toner.
JP-A No. SHO 62-111489 also discloses a cleaning device. The cleaning device disclosed in this publication includes a blade that scraps off the toner and a vibrator that vibrates the blade. Any toner or foreign particles remaining on the blade fall down as the blade vibrates.
JP-A No. HEI 6-51673 also discloses a cleaning device. The cleaning device disclosed in this publication includes a vibrator that comes in contact with a photoreceptor that caries the toner image. The vibrator vibrates the photoreceptor.
JP-A No. HEI 11-30938 also discloses a cleaning device. The cleaning device disclosed in this publication includes a blade, one end of which is fixed, that scraps off the toner and a vibrator that vibrates the fixed end of the blade. Any toner or foreign particles remaining on the photoreceptor fall down as the photoreceptor vibrates.
JP-A No. SHO 60-131547, JP-A No. HEI 6-148941, and JP-A No. HEI 8-254873 teach to use toner that includes the spherical toner and a toner containing irregular-shaped particles (hereinafter, “irregular toner”).
JP-A No. 2001-188452 requires the cleaning brush. This causes the size and the cost of the image forming device to increase. Moreover, it is quite difficult to pulverize the toner made of resin. Even if the toner can be pulverized, the toner damages the surface of the image carrier. The image quality degrades if the image carrier is damaged.
JP-A No. 2000-267536 teaches to use the mixture of powdered lubricant and toner having irregular-shaped particles and an average particle size smaller than that of the spherical toner. However, as the mixture is used, the image quality degrades in comparison when the spherical toner is used.
JP-A No. SHO 62-111489, JP-A No. HEI 6-51673, and JP-A No. HEI 11-30938 teach to use a vibrator to vibrate either the blade or the image carrier. However, these cleaning devices do not cope with a mechanism of occurrence of faulty cleaning for spherical toner, and therefore, the toner is not scrapped off completely (hereafter, “faulty cleaning”).
The inventors of the present invention studied why the faulty cleaning takes place in the cleaning blade of a counter type when the spherical toner was used. Their study showed that the reasons are as follows.
FIG. 53 illustrates a typical cleaning device of the counter type that scraps off the toner remaining (hereafter, “residual toner”) on an image carrier 111. A metal holder 100 holds a cleaning blade 101 in the following manner. That is, an edge of the cleaning blade 101 touches a surface of the image carrier 111, the cleaning blade 101 makes an acute angle θ with a tangent to the surface of the image carrier 111 in an opposite direction (i.e., counter direction) with respect to the direction of rotation (hereafter, “rotating direction”) A of the image carrier. That is, the flat part 101c of the cleaning blade 101 and the surface of the image carrier 111 form an angle. The edge of the cleaning blade 101 is pressed against the image carrier 111 for a distance d.
The cleaning blade 101 is made of elastic material with polyurethane rubber as the main component. The cleaning blade 101 generally has JISA hardness of 65 degrees to 70 degrees, thickness of about 1.5 millimeters to 2.0 millimeters, free length (i.e., length from the metal holder 100 to the edge of the cleaning blade 101) of 8 millimeters to 15 millimeters, and the angle θ is 20 degrees to 30 degrees.
When the image carrier 111 rotates, as illustrated in FIG. 54, an edge part 101b of a cut face 101a of the cleaning blade 101 is pulled in the rotating direction A, as the cleaning blade 101 is made of the elastic material, due to a frictional force with the image carrier 111. As a result, the cut face 101a deforms in the form of a curl. Because of such distortion, a wedge-shaped space is produced between the cut face 101a and the image carrier 111.
FIG. 55 is an enlarged view of a nip portion N shown in FIG. 54. Considering that the toner is the irregular toner, the toner Ta enters into the wedge-shaped space. The toner particles make a stick-slip movement in this space.
The stick-slip movement is explained with reference to FIG. 56. When a nip formed by the blade toward the image carrier (“blade nip”) is fixed on the moving surface of the image carrier, the blade nip is forcefully expanded in the rotating direction of the image carrier 111 as shown by a broken line in FIG. 56. When the blade nip is expanded to a certain position, the repulsive force of the blade is increased to such an extent that the static frictional force and the repulsive force are balanced. At this point in time, the blade nip slides over the surface of the image carrier. In a state where sliding occurs between the blade nip and the image carrier, the coefficient of dynamic friction is smaller than the coefficient of static friction, and thereby the blade nip returns to the original direction (direction indicated by a solid line) while sliding over the surface of the image carrier. By the restoring force of the stick-slip movement repeatedly performed (the range is indicated by SP), the toner Ta staying in the wedge-like nip undergo the force of returning in the direction opposite to the preceding direction of the image carrier 111, and is cleaned.
On the other hand, if the toner is the spherical toner, as illustrated in FIG. 57, the toner Tb does not remain in wedge-shaped space because the toner Tb has spherical shape. The toner Tb rotates in this space due to the frictional force with the image carrier 111. Therefore, the toner Tb moves along with the image carrier 111 while rotating in the direction reverse to the rotating direction A of the image carrier 111, and passes through the nip between the cleaning blade 101 and the image carrier 111. This results into faulty cleaning.
Moreover, the toner Tb that passes through the nip function as a lubricant and, as illustrated in FIG. 58, reduces the frictional force between the cleaning blade 101 and the image carrier 111. This makes the tip of the cleaning blade 101 flat (as against curly). As a result, the toner Tb does not undergo the stick-slip movement. This results into faulty cleaning.
The fine toner more easily enters into the nip as illustrated in FIG. 57. Moreover, even if the particles are irregular shapes, they do not have so many edges. Therefore, the fine toner easily passes through the nip.
The inventors further studied on the mechanism of occurrence of faulty cleaning for the spherical and fine toner based on the mechanism of the occurrence previously found by the inventors. As a result of the study, the inventors have found that a new mechanism different from the conventional cleaning mechanism allows the spherical and fine toner to be cleaned.
In the blade cleaning method (cleaning method using the cleaning blade), it is generally known that the toner cleaning capability is enhanced, in other words, the capability of sliding contact is increased and therefore the frictional resistance between the blade and the image carrier tends to be increased.
Particularly, if no residual toner remains on the image carrier after toner transfer, the frictional resistance between the blade (edge) part and the image carrier extremely increases. If the frictional resistance is very high, the blade is pulled to the image carrier, and a so-called “warping” occurs, which may prevent the cleaning capability from being fully delivered.
In the cleaning method using the blade, the image carrier is slid by the rubber blade as explained above to scrape the toner off the image carrier. Therefore, the edge front of the rubber blade is deformed by the frictional resistance between the image carrier and the rubber blade, and a fine wedge-like space is formed between the two. Since forming the wedge-like region is important for the cleaning, formation of the blade edge is largely related to the cleaning capability.
However, if the frictional resistance between the blade edge and the image carrier is too high, the “warping” occurs, and in addition to this, the edge part essential to the cleaning may be chipped. Therefore, it is preferable to maintain the edge part while minimum frictional resistance required for the cleaning is ensured and maintained in terms of durability. However, the cleaning unit using the conventional cleaning blade has a problem such that the unit cannot ensure and maintain such minimum frictional resistance required for the cleaning.
Furthermore, the inventors have found that deficiency also occurs to the image carrier other than the chipping of the blade edge. The deficiency is caused by an increase in the frictional resistance between the blade edge and the image carrier.
An example of the deficiency includes filming such that the toner is elongated by the blade over the surface of the image carrier to form a toner film layer on the image carrier, and film scraping of a photoreceptive layer forming the image carrier. Since the filming is formed on the photoreceptive layer, the filming becomes an obstacle when an accurate image is formed on the photoreceptor. Further, the film scraping causes the durability of the image carrier to be reduced. Therefore, it is necessary that the deficiencies are prevented to occur as less as possible. There is also a case where the film scraping is proceeding when the filming occurs, and in this case, sometimes, only the film scraping seems to occur, depending on the case.
It has been found that these deficiencies occur in the image forming apparatus using the cleaning blade because the frictional resistance between the blade and the image carrier is high.